1. Field of the Invention
The present invention relates to chemical process towers and, more particularly, but not by way of limitation, to a downcomer-tray assembly for maximizing efficiency in a trayed tower.
2. History of Related Art
Distillation columns are utilized to separate selected components from a multi component stream. Successful fractionation in the column is dependent upon intimate contact between liquid and vapor phases. Some columns use vapor and liquid contact devices such as trays.
The above-referenced trays are generally installed on support rings within the tower and have a solid tray or deck with a plurality of apertures in an "active" area. Liquid is directed onto the tray by means of a vertical channel from the tray above. This channel is referred to as the downcomer. The liquid moves across the active area and exits through a similar downcomer. The location of the downcomers determine the flow pattern of the liquid. Vapor ascends through the apertures in the trays and contacts the liquid moving across the tray. The liquid and vapor mix in the active area and fractionation occurs. It is the active area of the tray that is of critical concern.
The maximum fractionation capacity of the tray generally increases with an increase in the active or bubbling area. Maximum utilization of active area of a tray is an important consideration to chemical process tower design. Regions of the tray which are not effectively used for vapor-liquid contact can reduce the fractionation capacity and efficiency of the tray. Therefore, there is a need for devices and methods that optimize the active area of a fractionation tray in a chemical process tower.
It is well known that the concentration-difference between the vapor and the liquid is the driving force to effect mass transfer. Said concentration-difference can be effected in many ways; some reducing fractionation efficiency. When operating pressure is such as to produce a vapor density above about 1.0 lbs/cu. ft., there is the possibility that some amount of vapor bubbles are commingled or entrained with the downcomer incoming liquid. For example, as operating pressure increases due to an increase in the vapor concentration, descending liquid begins to absorb vapor as it moves across a tray. This is above that normally associated as dissolved gas as governed by Henry's Law and represents much larger amounts of vapor bubbles that are commingled or "entrained" with the liquid. This vapor is not firmly held and is released within the downcomer, and, in fact, the majority of said vapor must be released, otherwise the downcomer can not accommodate the liquid/vapor mixture and will flood thus preventing successful tower operation.
Similarly, an exothermic reaction in the downcomer will generate vapors from the equilibrium mixture, which also will be released. For conventional trays, the released vapor will oppose the descending frothy vapor/liquid mixture flowing into the downcomer. In many cases, such opposition leads to poor tower operation and premature flooding. Therefore, there is a need for devices and methods that facilitate the release of vapor entrained in the liquid within a downcomer of a chemical process tower.
Another serious problem which manifests itself in such operational applications is entrainment of liquid droplets in the ascending vapor. This phenomenon, which is virtually the opposite of the above vapor entrainment, can prevent effective vapor liquid contact. Liquid entrainment is, in one sense, a dynamic flow condition. High velocity vapor flow can suspend descending liquid droplets and prevent their effective passage through the underlying froth mixture zone. It is particularly difficult to prevent this problem when the tower applications require high volume vapor flow in a direction virtually opposite to that of high volume, descending liquid flow. Therefore, there is a need for devices and methods that will reduce the liquid entrained in the vapor within a chemical process tower.
Efficiency of a tray is also reduced when vapor ascending through the process column is allowed to by-pass the active area of a tray. One area where vapor can bypass the active area of a tray is the downcomer. When vapor intended for the active area of the tray unintentionally passes through the downcomer the efficiency of the active area in the tray is reduced. Also, vapor unintentionally passing through the downcomer will reduce the flow of liquid through the downcomer and potentially cause a backup of the liquid flowing through the process column. Therefore, there is a need for devices and methods that reduce the amount of vapor that flows through a downcomer.
Efficiency of the active area in a tray is also influenced by the flow of liquid across the active area. At the initial point of contact of liquid from a downcomer onto the tray, the flow of the liquid is not typically a flow characteristic that provides optimum efficiency for the active area of a tray. Therefore, there is a need for devices and methods that assist in the change of flow characteristics of fluid from a downcomer onto the active area of a tray and also across the active area of a tray. The present invention provides such a method and apparatus for maximizing mass transfer efficiency in chemical process towers.